Femtosecond laser-assisted selective holding with ultra-low power for direct manipulation of biological species

SignificanceOptical tweezers (OTs) have emerged as an essential technique for manipulating nanoscopic particulates and biological specimens with sub-micron precision and have revolutionized various fields, including biology and colloidal physics. However, traditional optical trapping techniques often rely on moderate- to high-power continuous wave (CW) lasers, which can introduce unwanted thermal effects and photodamage to delicate samples. An innovative alternative has emerged through the utilization of femtosecond (fs) lasers at ultra-low average powers on the order of tens of microwatt. Unexpectedly overlooked until now, this method enables the direct trapping and manipulation of cells without relying on functionalized spheres.AimWe aim to compare the trap stiffness of CW and fs lasers in an unexplored average power regime (sub-1 mW) on cells within the intermediate-size regime.ApproachA CW or fs laser is used to trap cells in an inverted microscope setup. We trap five different pathogenic bacteria with different morphologies to compare trap stiffness.ResultsWe find that fs laser-assisted selective holding with ultra-low power (FLASH-UP) exhibits five times greater trap stiffness than CW-based OTs and can trap at lower intensities. Furthermore, we demonstrate that FLASH-UP does not impact cell motility.ConclusionsFLASH-UP displays higher trap stiffness at average powers below 1 mW and does not impact cell functionality. These results pave the way for ultra-low-power trapping of cells for applications in sorting, bio-sensing, in vivo cell manipulation, and single-cell analysis.